Storage devices are equipped with a large number of memory devices for storing data, and a storage controller for controlling the memory devices, and the object thereof is to provide a large capacity of data storage space to a computer.
Heretofore, HDDs (Hard Disk Drives) are installed as memory devices, but recently, memory devices (such as SSDs: Solid State Drives) having nonvolatile semiconductor memories (such as FMs: Flash Memories) are attracting attention as new storage media that can be used in replacement of HDDs.
Generally, many SSDs are equipped with a plurality of NAND type FM chips, and the FM chips perform reading and writing of data in units called pages. Since the FM cannot directly overwrite the data stored in a page, data must be erased once to store new data. Erasing of data is executed to a group of pages called a block. Further, a group of blocks called a die is constructed within the FM. There is an upper limit in the number of times that a block can be erased. In other words, the FM has a life, and the SSD including the large number of FMs also has a life. For example, if the FM reaches its end of life, reading and writing of data may not be performed correctly. If data cannot be read correctly, the SSD will be in a state where stored data is lost.
Further, the quality of the FMs may be dispersed in units of dies and chips. If the quality of the die is not good, failure may occur by the die reaching its life even before the number of erase counts reaches a predetermined target count, and the die becomes unusable. For example, SSD provides a storage capacity (hereinafter called logical capacity) to the user. However, if failure occurs to a part of the FM chips and the capacity cannot be maintained, the SSD will be blocked even if there are other FM chips having high quality and are in a usable state.
As described, if the SSD becomes unusable earlier than the predetermined period of time, the frequency of replacement increases, requiring costs for additional purchase or maintenance of SSDs, which may lead to the increase of TCO. Further, if the whole SSD is blocked when failure occurs to only a part of the FM chips, the large number of remaining usable FM chips will be wasted.
Patent Literature 1 discloses a technique that enables to block only the area where failure has occurred, instead of blocking the whole SSD in which FM chip failure has occurred, to allow continuous use of the SSD.